1. Field of the Invention
The present invention relates to a cyclometalated transition metal complex and an organic electroluminescent device using the same. More particularly, the present invention relates to a cyclometalated transition metal complex that can emit light ranging from a blue region to a red region from a triplet metal-to-ligand charge-transfer (MLCT) state and to an organic electroluminescent device using the cyclometalated transition metal complex as an emitting organic layer material.
2. Description of the Related Art
Organic electroluminescent (EL) devices are self-emission displays that emit light by recombination of electrons and holes in a thin layer (hereinafter, referred to as “organic layer”) made of a fluorescent or phosphorescent organic compound when a current is applied to the organic layer. The organic EL devices have advantages such as lightweight, simple constitutional elements, an easy fabrication process, high image quality, and a wide viewing angle. Furthermore, high color purity and perfect creation of dynamic images are ensured. In addition, the organic EL devices have electrical properties suitable for portable electronic equipment such as low power consumption and low driving voltage.
A common organic EL device has a sequentially stacked structure of an anode, a hole transport layer, a light emission layer, an electron transport layer, and a cathode, on an upper surface of a substrate. The hole transport layer, the light emission layer, and the electron transport layer are organic layers made of organic compounds. The organic EL device with the above-described structural feature is driven as follows. When a voltage is applied to the anode and the cathode, holes from the anode are transferred to the light emission layer via the hole transport layer. On the other hand, electrons from the cathode are transferred to the light emission layer via the electron transport layer. Carriers recombine at the light emission layer to generate excitons. By the radiative decay of the excitons, light emission occurs at the wavelength corresponding to the bandgap of a material.
A material for the light emission layer in the organic EL device is divided into a fluorescent material using a singlet exciton and a phosphorescent material using a triplet exciton according to emission mechanism. The light emission layer is formed of the fluorescent or phosphorescent material alone or an appropriate host material doped with the fluorescent or phosphorescent material. Singlet excitons and triplet excitons are formed in the host during electronic excitation. At this time, a statistical ratio of the singlet excitons to the triplet excitons is 1 to 3.
An organic EL device including a light emission layer made of a fluorescent material has a disadvantage in that triplet excitons formed in a host are wasted. On the other hand, an organic EL device including a light emission layer made of a phosphorescent material has an advantage of 100% internal quantum efficiency since both singlet excitons and triplet excitons can be utilized. In this respect, a light emission layer made of a phosphorescent material can achieve significantly high emission efficiency, relative to a light emission layer made of a fluorescent material.
When a heavy metal such as Ir, Pt, Rh, and Pd is introduced into an organic molecule, the heavy atom effect leads to spin-orbital coupling, whereby a triplet state and a singlet state are mixed. Therefore, a forbidden transition is induced, which allows efficient phosphorescent emission even at room temperature.
Recently, there have been developed high-efficiency, green and red phosphorescent materials with 100% internal quantum efficiency.
As high-efficiency phosphorescent materials, there have been reported various materials based on transition metal compounds containing transition metal such as Osmium, iridium and platinum. To date, green and red phosphorescent materials satisfying characteristics necessary for high-efficiency full-color displays or low power consumption and white light emission applications have been reported. However, efficient and reliable blue phosphorescent materials have not been developed, which is a significant obstruction to development of phosphorescent full-color devices.
In view of this problem, a blue-emitting material has been developed (WO 02/15645 A1, US 2002/0064681 A1 entitled Luminescence device, display apparatus and metal coordination compound to Takiguchi et al., and published on May 3, 2002). Furthermore, there has been developed an organometallic complex having a bulky functional group capable of increasing a HOMO-LUMO difference by a molecular geometrical change or a functional group (e.g., cyano group) with high ligand field strength. In addition, there have been developed an iridium complex represented by formula Ir(ppy)2P(ph)3Y (Y═Cl or CN) (US2002/0182441 A1 entitled Organometallic compounds and emission-shifting organic electrophosphorescence to Lamansky et al., and published on Dec. 5, 2002) and an iridium (III) complex having a cyclometalated ligand, a chelating diphosphine, chlorine, and cyano group (US 2002/0048689 A1 entitled Light-emitting device and iridium complex to Igarashi et al., and published on Apr. 25, 2002).